In a digital color image forming device, it is general to use three color materials of yellow (Y), magenta (M), and cyan (C), or four color materials of the foregoing three and black. Furthermore, a technique of using more than four color materials is recently often used to obtain good image quality.
In the case where an image is formed with color materials of many colors, a full-color image in a good state can be obtained by precisely adjusting respective printed quantities of the color materials used. Therefore, mixing of the color materials before printing leads to serious deterioration in image quality, and to prevent this problem, it is usual to prepare an equal number of image forming stations to the number of the color materials.
Furthermore, since a change in relative color material printed positions leads to a change in superimposition of the printed color materials, a color resulting on the superimposition of the color materials is sometimes recognized by human eyes as different from a color resulting on the superimposition of the same quantities of the same color materials. Therefore, by precisely adjusting relative positions of the image forming stations, and further, by maintaining the adjusted positions without changes as time elapses, it is possible to stably maintain the relative color material printed positions at all times. Thus, keys to high image quality are to precisely adjust printed quantities of the color materials and to improve precision in positioning the image forming stations.
Resolution of an image forming device such as a copying machine or a printer of these days is at least about 400 dpi (dots/inch), and sometimes about 600 dpi in the case of a device with high resolution. Incidentally, if a device has a resolution of 300 dpi, a size of each pixel of the device is about 85 .mu.m.
For example, assume that it is requested to form on paper one line extending in a proceeding direction of paper (sub scanning direction) (hereinafter referred to as a vertical line) by using an image forming device wherein two image forming stations are juxtaposed in the paper proceeding direction and an image is formed by use of the two image forming stations. If relative positions of the image forming stations do not agree but differ by one pixel in a direction crossing the paper proceeding direction (main scanning direction), not one but two vertical lines are outputted. This is far from high image quality. Even if the position difference between the image forming stations is suppressed to 1/2 pixel, not one line but 1.5 line is obtained, and this is also far from high image quality.
In each image forming station as well, inappropriate relative positions of an image optical scanning recording section and a photosensitive body lead to the following drawbacks; in the case where a space between the image optical scanning recording section and the photosensitive body is too wide, an image which is expanded in the main scanning direction is outputted; and in the case where the space therebetween is too narrow, an image which is reduced in the main scanning direction is outputted.
Therefore, clear from the foregoing description, the precision in positioning the image forming stations and the image quality are closely related, and mechanical precision in .mu.m order is required to achieve high image quality.
It is, however, difficult to achieve the position precision in Am order with the current mechanical techniques, and even if it is possible, it costs extremely high. Furthermore, considering that this has to be maintained for a long time (life time of a product is 3 to 5 year in average) and taking environmental changes into consideration, it is almost impossible.
As an arrangement with which the foregoing problem is solved, the Japanese Publication for Laid-Open Patent Application No. 64473/1988 (Tokukaisho 63-64473) and the Japanese Publication for Laid-Open Patent Application No. 191170/1988 (Tokukaisho 63-191170) disclose an arrangement wherein each image optical scanning recording section in each image forming station is made to have a plurality of reference clock signals differing in frequency and phase, and the most suitable reference clock signal is selected and used from among the prepared reference clocks, so that a mechanical offset is eliminated.
According to the arrangement disclosed by the foregoing publications, however, many reference clock signals have to be prepared so as to cancel the mechanical offset. In other words, the precision in cancelling the mechanical offset is proportional to the number of the prepared reference clock signals. As higher image quality is demanded, the circuit scale expands while the circuit structure becomes more complicated. Besides, stepless adjustment of the offset requires an infinite number of reference clock signals, which is practically impossible. Therefore, precision in correction is only achieved to some extent since the circuit scale is limited.